1. Field of the Invention
The present invention relates to a resin sealed semiconductor device in which a semiconductor element is mounted on a die pad of a lead frame and sealed in a resin.
2. Discussion of Background
In a conventional resin sealed semiconductor device, occurrence of package cracking caused by separation of a metal die pad and resin poses a serious issue. Therefore, various proposals have conventionally been carried out to prevent package cracking. FIG. 4 shows an example thereof wherein numeral 1 designates a die pad of a lead frame, numeral 2 designates a semiconductor element, numeral 3 designates solder bonding the semiconductor element 2 to the die pad 1 and numeral 4 designates a number of dimples on the back face of the die pad 1. In the example of FIG. 4, an anchoring effect with respect to a sealing resin (not shown) is produced by the dimples 4 whereby the separation of the sealed resin from the die pad becomes much more difficult and package cracking can be prevented to some degree. However, in this semiconductor device the formation of the dimples 4 can be performed only by an etching process and accordingly, the production cost of the lead frame including the die pad 1 is increased.
Therefore, a semiconductor device has conventionally been proposed as shown in FIG. 5 where an anchoring effect is achieved between the die pad 1 and a sealing resin (not shown) by forming a plurality of through holes 5 in the die pad 1 (for example, Japanese Unexamined Patent Publication No. Sho. 56-104459, Japanese Unexamined Patent Publication No. Sho. 63-249341, Japanese Unexamined Patent Publication No. Hei 2-246349 and the like). In the semiconductor device including a plurality of through holes 5 in the die pad 1, the through holes 5 can be formed by a punching process and, therefore, the lead frame can be manufactured at a low cost.
However, in such a conventional semiconductor device, when a paste material 6 or solder is used for bonding the semiconductor element 2 to the die pad 1, as shown in FIG. 5, the paste material 6 or solder sinks into the through holes 5 of the die pad 1 and the sealing resin does not fill up the through holes 5. As a result, the anchoring effect by the through holes 5 is not provided.
Further, there is known a conventional semiconductor device in which the through holes 5 are located only at peripheral portions of the die pad 1 where the semiconductor element 2 is not mounted to prevent the paste material 6 or solder from sinking into the through holes 5 of the die pad 1. However, in such a semiconductor device, the through holes are not located at a portion of the die pad 1 corresponding to the back face of the semiconductor element 2 and accordingly, separation is apt to occur between the portion of the die pad 1 where the through holes are not present and the sealing resin.
Further, there is known a conventional semiconductor device in which the base material 6 or solder is prevented from sinking into the through holes 5 by providing comparatively wide mutual intervals between the through holes 5 and by interspersing the paste material 6 or solder the intervals. However, in such a semiconductor device, it is necessary to provide comparatively wide intervals between the through holes 5 and accordingly, the number of the through holes 5 is decreased and separation of the sealing resin from the die pad 1 is apt to occur. Further, the sealing resin which has entered the through holes 5 is brought into direct contact with the back face of the semiconductor element 2 and therefore, separation of the sealing resin from the back face of the semiconductor element 2 is apt to occur which gives rise to easy cracking of the package.